FIG. 18 is a cross-sectional view of conventional circuit board 10. Wiring 20 formed from a lead frame is embedded in resin 30 to form circuit board 10. If required, metallic plate 40 for heat dissipation is attached onto a rear surface of circuit board 10 (see, for example, PTL 1).
In circuit board 10, as a metallic material constituting wiring 20, tough-pitch copper or a wiring material for use in flexible wiring boards is used. The tough-pitch copper and a wiring material have excellent flexibility. However, the flexibility of circuit board 10 is insufficient yet.
When metallic plate 40 is removed from circuit board 10, circuit board 10 can be thinner. However, in circuit board 10 from which metal 40 is removed, when a filling rate of a ceramic filler contained in resin 30 is increased for the purpose of bringing a coefficient of thermal expansion of resin 30 close to that of a semiconductor, the flexibility of circuit board 10 decreases. As a result, circuit board 10 may be broken easily upon bending.
On the other hand, a wafer-level chip size package or the like has been demanded in the market. In a cured product of a liquid resin composition, a circuit board and the like which are used in the package, a low coefficient of thermal expansion, a certain level or higher of stiffness and a certain level or higher of flexibility of a thinned product of the cured product, the circuit board and the like are important. As the package for use in the above-mentioned applications, a wafer-level chip size package has been proposed (see, for example, PTL 2).
For the above-mentioned purposes, a liquid sealing resin composition containing a silicone-modified liquid epoxy resin, a liquid polyphenol, a curing accelerator and an inorganic filler has been proposed (see, for example, PTL 3).